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We present a series of statistical tests using homogeneous data and measurements for a sample of Seyfert galaxies. These galaxies were selected from mostly isotropic properties, their far infrared fluxes and warm infrared colors, which provide a considerable advantage over the criteria used by most investigators in the past, like ultraviolet excess. Our results provide strong support for a Unified Model in which Seyferts 2’s contain a torus seen more edge-on than in Seyferts l’s and show that previous results showing the opposite were most likely due to selection effects.

This paper explores how the stochastic accretion of planetesimals onto white dwarfs would be manifested in observations of their atmospheric pollution. Archival observations of pollution levels for unbiased samples of DA and non-DA white dwarfs are used to derive the distribution of accretion rates, confirming that rates become systematically lower as sinking time is decreased, with no discernable dependence on cooling age. The accretion rates expected from planetesimals that are all the same mass (ie, a mono-mass distribution) are explored both analytically and using a Monte Carlo model, quantifying how measured accretion rates inevitably depend on sinking time, since different sinking times probe different times since the last accretion event. However, that dependence is so dramatic that a mono-mass distribution can be excluded. Consideration of accretion from a broad distribution of planetesimal masses uncovers an important conceptual difference: accretion is continuous (rather than stochastic) for planetesimals below a certain mass, and the accretion of such planetesimals determines the rate typically inferred from observations; smaller planetesimals dominate the rates for shorter sinking times. A reasonable fit to the observationally inferred accretion rate distributions is found with model parameters consistent with a collisionally evolved mass distribution up to Pluto-mass, and an underlying accretion rate distribution consistent with that expected from descendants of debris discs of main sequence A stars. With these parameters, while both DA and non-DA white dwarfs accrete from the same broad planetesimal distribution, this model predicts that the pollution seen in DAs is dominated by the continuous accretion of 35km objects (though the dominant size varies between stars by around an order of magnitude from this value)..., 23 pages, MNRAS in press

In this Letter, we analyse the distributions of stellar ages in giant molecular clouds (GMCs) in spiral arms, interarm spurs and at large galactic radii, where the spiral arms are relatively weak. We use the results of numerical simulations of galaxies, which follow the evolution of GMCs and include star particles where star formation events occur. We find that GMCs in spiral arms tend to have predominantly young (

In a recent paper, Elmegreen (2000) has made a cogent case, from an observational point of view, that the lifetimes of molecular clouds are comparable to their dynamical timescales. If so, this has important implications for the mechanisms by which molecular clouds form. In particular we consider the hypothesis that molecular clouds may form not by {\it in situ} cooling of atomic gas, but rather by the agglomeration of the dense phase of the interstellar medium (ISM), much, if not most, of which is already in molecular form., 6 pages, no figures, accepted on 20 June 2001 for publication in MNRAS

We present a simple physical mechanism that can account for the observed stellar mass spectrum for masses $\ms \simgreat 0.5 \solm$. The model depends solely on the competitive accretion that occurs in stellar clusters where each star's accretion rate depends on the local gas density and the square of the accretion radius. In a stellar cluster, there are two different regimes depending on whether the gas or the stars dominate the gravitational potential. When the cluster is dominated by cold gas, the accretion radius is given by a tidal-lobe radius. This occurs as the cluster collapses towards a $\rho\propto R^{-2}$ distribution. Accretion in this regime results in a mass spectrum with an asymptotic limit of $\gamma=-3/2$ (where Salpeter is $\gamma=-2.35$). Once the stars dominate the potential and are virialised, which occurs first in the cluster core, the accretion radius is the Bondi-Hoyle radius. The resultant mass spectrum has an asymptotic limit of $\gamma=-2$ with slightly steeper slopes ($\gamma\approx-2.5$) if the stars are already mass-segr egated. Simulations of accretion onto clusters containing 1000 stars show that as expected, the low-mass stars accumulate the majority of their masses during the gas dominated phase whereas the high-mass stars accumulate the majority of their massed during the stellar dominated phase. This results in a mass spectrum with a relatively shallow $\gamma\approx 3/2$ power-law for low-mass stars and a steeper, power-law for high-mass stars $ -2.5\simless\gamma\le -2$. This competitive accretion model also results in a mass segregated cluster.

We study protoplanetary disc evolution assuming that angular momentum transport is driven by gravitational instability at large radii, and magnetohydrodynamic (MHD) turbulence in the hot inner regions. At radii of the order of 1 AU such discs develop a magnetically layered structure, with accretion occurring in an ionized surface layer overlying quiescent gas that is too cool to sustain MHD turbulence. We show that layered discs are subject to a limit cycle instability, in which accretion onto the protostar occurs in bursts with an accretion rate of 10^{-5} solar masses / yr, separated by quiescent intervals where the accretion rate is 10^{-8} solar masses / yr. Such bursts could lead to repeated episodes of strong mass outflow in Young Stellar Objects. The transition to this episodic mode of accretion occurs at an early epoch (t < 1 Myr), and the model therefore predicts that many young pre-main-sequence stars should have low rates of accretion through the inner disc. At ages of a few Myr, the discs are up to an order of magnitude more massive than the minimum mass solar nebula, with most of the mass locked up in the quiescent layer of the disc at around 1 AU. The predicted rate of low mass planetary migration is reduced at the outer edge of the layered disc, which could lead to an enhanced probability of giant planet formation at radii of 1-3 AU., MNRAS, in press

We present the study of the relative angle between the accretion disk (or radio jet) and the galaxy disk for a sample of Seyfert galaxies selected from a mostly isotropic property, the 60$\mu$m flux. For each galaxy we have a pair of points ($i$,$\delta$), which are the inclination of the galaxy relative to the line of sight and the angle between the jet projected into the plane of the sky and the host galaxy major axis, respectively. This data is combined with a statistical technique to determine the distribution of $\beta$ angles {\it in 3 dimensions}, the angle between the jet and the host galaxy plane axis. The analysis of the data, not differentiating between Seyfert 1's and 2's, showed that the observed distribution of $i$ and $\delta$ values can be well represented by a homogeneous $\beta-$distribution in the range $0^{\circ}\leq\beta\leq90^{\circ}$. However, when we distinguished between Seyfert 1's and 2's, the models could not represent Seyfert 1's adequately. It was necessary to introduce viewing angle restrictions, that a galaxy can only be recognized as a Seyfert 1 if the angle between the jet and the line of sight ($|\phi|$) is smaller than a given angle $\phi_c$ and that the galaxy inclination $i$ is smaller than an angle $i_c$, in order to have statistically acceptable models. This is a direct and independent confirmation of the underlying concepts of the Unified Model. We discuss several ways to explain the misalignment between the accretion disk axis and the host galaxy disk axis., Comment: 58 pages, 16 figures (included) and 5 tables. To appear in The Astrophysical Journal

T Tauri stars are young pre–main–sequence stars and were first identified because of their variability. They emit excess radiation, relative to normal stars, at both long (infrared) and short (ultraviolet) wavelengths, and also emit at X–ray and radio wavelengths. Early models for both the excess radiation and variability in terms of strongly enhanced magnetic chromospheric/coronal activity were not very successful. Once it was realized that most T Tauri stars lie at the centre of gaseous discs (left over from the process of star formation), it became widely accepted that the UV excess is caused by continued accretion, and the IR excess by radiation from the disc. More detailed investigations of this model have brought the wheel full circle with the realization that T Tauri stars do have strong magnetic activity, and that many of their properties can be accounted for in terms of interactions between the stellar fields and the surrounding discs.

Precessing accretion discs have long been suggested as explanations for the long periods observed in a variety of X-ray binaries, most notably Her X-1/HZ Her. We show that an instability of the disc's response to the radiation reaction force from the illumination by the central source can cause the disc to tilt out of the orbital plane and precess in something like the required manner. The rate of precession and disc tilt obtained for realistic values of system parameters compare favourably with the known body of data on X-ray binaries with long periods. We explore other possible types of behaviour than steadily precessing discs that might be observable in systems with somewhat different parameters. At high luminosities, the inner disc tilts through more than 90 degrees, i.e. it rotates counter to the usual direction, which may explain the torque reversals in systems such as 4U 1626-67., Comment: submitted to MNRAS, 17-Dec-97, revised submit 2-Nov-98. 15 pages LaTeX, 11 postscript figures in-text

We present numerical calculations of the evolution of an accretion disk that is irradiated by a central source. The parameters chosen represent those of the central star of a planetary nebula. We show that an irradiation-induced warping can occur in the disk and that it is accompanied by a wobbling motion in both the inclination and azimuthal directions. Since jets are assumed to be perpendicular to the inner disk, this motion is capable of producing the "point-symmetric" morphology observed in some nebulae.

We consider the thermal and dynamical balance in dense molecular cloud cores. We point out that because such cores are heated throughout their volumes by cosmic rays, but cool radiatively by means of optically thick lines, they are likely to be dynamically unstable and at best in only a form of quasi-static equilibrium. We also draw attention to the possibility that if such cores undergo an external impulse that is large enough to produce a velocity difference ~v;::S c., where Cs is the thermal sound speed, then the cooling rate rises suddenly and pressure support drops, which leads to gravitational collapse and fragmentation.

We predict the existence of dusty disks and possibly CO planets around massive white dwarfs. We show that the thermal emission from these disks should be detectable in the infrared. The planets may also be detectable either by direct IR imaging, spectroscopy, or using the pulsations of the white dwarfs., 8 pages, 1 figure; accepted ApJL

We present a detailed study of the evolution of GMCs in a galactic disc simulation. We follow individual GMCs (defined in our simulations by a total column density criterion), including their level of star formation, from their formation to dispersal. We find the evolution of GMCs is highly complex. GMCs often form from a combination of smaller clouds and ambient ISM, and similarly disperse by splitting into a number of smaller clouds and ambient ISM. However some clouds emerge as the result of the disruption of a more massive GMC, rather than from the assembly of smaller clouds. Likewise in some cases, clouds accrete onto more massive clouds rather than disperse. Because of the difficulty of determining a precursor or successor of a given GMC, determining GMC histories and lifetimes is highly non-trivial. Using a definition relating to the continuous evolution of a cloud, we obtain lifetimes typically of 4-25 Myr for >10^5 M$_{\odot}$ GMCs, over which time the star formation efficiency is about 1 %. We also relate the lifetime of GMCs to their crossing time. We find that the crossing time is a reasonable measure of the actual lifetime of the cloud, although there is considerable scatter. The scatter is found to be unavoidable because of the complex and varied shapes and dynamics of the clouds. We study cloud dispersal in detail and find both stellar feedback and shear contribute to cloud disruption. We also demonstrate that GMCs do not behave as ridge clouds, rather massive spiral arm GMCs evolve into smaller clouds in inter-arm spurs., 15 pages, 16 figures, accepted for publication in MNRAS

Motivated by recent observations of short-timescale variations in the infrared emission of circumstellar disks, we propose that coronal mass ejections can remove dust grains on timescales as short as a few days. Continuous monitoring of stellar activity, coupled to infrared observations, can place meaningful constraints on the proposed mechanism., Comment: accepted to Astrophysical Journal Letters

Standard, planar accretion discs operate through a dissipative mechanism, usually thought to be turbulent, and often modelled as a viscosity. This acts to take energy from the radial shear, enabling the flow of mass and angular momentum in the radial direction. In a previous paper we discussed observational evidence for the magnitude of this viscosity, and pointed out discrepancies between these values and those obtained in numerical simulations. In this paper we discuss the observational evidence for the magnitude of the dissipative effects which act in non--planar discs, both to transfer and to eliminate the non--planarity. Estimates based on the model by Ogilvie (1999), which assumes a small--scale, isotropic viscosity, give alignment timescales for fully ionized discs which are apparently too short by a factor of a few compared with observations, although we emphasize that more detailed computations as well as tighter observational constraints are required to verify this conclusion. For discs with low temperature and conductivity, we find that the timescales for disc alignment based on isotropic viscosity are too short by around two orders of magnitude. This large discrepancy suggests that our understanding of viscosity in quiescent discs is currently inadequate., Comment: 7 pages, no figures. MNRAS, in press

We consider a thin accretion disc of half-thickness H, vertically threaded by a magnetic field. The field is due to contributions from both the disc current and an external current (giving rise to a uniform external field). We derive an integro-differential equation for the evolution of the magnetic field, subject to magnetic diffusivity η and disc accretion with radial velocity v r . The evolution equation is solved numerically, and a steady state is reached. The evolution equation depends upon a single, dimensionless parameter D=2η/(3H|v r |)=(R/H)(η/ν), where the latter equality holds for a viscous disc having viscosity ν. At the disc surface, field lines are bent by angle i from the vertical, such that tan i=1.52D −1

We propose two new means of identifying the main class of progenitors of Type Ia supernovae--single or double degenerate: (i) If the range of supernova properties is significantly determined by the range of viewing angles of non-spherically symmetric explosions, then the nature of the correlation between polarization and another property (for example, the velocity gradient) can be used to determine the geometry of the asymmetry and hence the nature of the progenitor, and (ii) in the double- but not in the single-degenerate case, the range in the observed properties (e.g., velocity gradients) is likely to increase with the amount of carbon seen in the ejecta., Comment: 10 pages, 2 figures; ApJL in press

Ultraviolet spectra of nova-like variables appear to be cooler than those of dwarf novae in outburst. This suggests that, once a steady-state accretion disc becomes established, it releases less of its energy thermally and more in other forms such as mass loss or magnetic flux. Here we use archival IUE data for the anti-dwarf nova TT Ari to show that its spectrum also is cooler at the end of a period in a high state than it is just after a minimum. Models such as that proposed by Tout & Pringle for magnetic viscosity in the disc could account for this kind of behaviour

We consider the linear stability of an isothermal, pressure-bounded, self-gravitating gas slab. Such a configuration is unstable at sufficiently long wavelengths and grows at rate ∼ √Gρ, where ρ is the density of the gas slab, but at high external pressure the nature of this instability is quite different from that of the usual Jeans instability. We develop an analytical model that reproduces the behaviour of the instability found numerically by Elmegreen & Elmegreen. They found, surprisingly, that the critical wavenumber for the onset of the instability is always of the order of the layer thickness, independently of the level of self- gravity and the external pressure

We investigate the effect on an accretion disc around one star of a parabolic fly-by of another. In the prograde, coplanar encounter, the disc is tidally stripped down to about one half of the periastron radius, and a substantial fraction of the stripped material is captured by the intruder. In the retrograde, coplanar encounter, the disc is essentially unaffected within periastron. We also consider an encounter in which the disc and orbit are initially orthogonal. Here the disc outside periastron is only partially tidally stripped, essentially no material is captured by the intruder, and the disc which remains is twisted. We comment briefly on the significance of the above results to models of binary star formation

The nearest accretion disc to us in space if not time was the protosolar nebula. Remnants of this nebula thus potentially offer unique insight into how discs work. In particular the existence of chondrules, which must have formed in the disc as small molten droplets, requires strong and intermittent heating of disc material. We argue that this places important constraints on the way gravitational energy is released in accretion discs, which are not met by current shearing--box simulations of MRI--driven dynamos. A deeper understanding of accretion energy release in discs may require a better model for these dynamos., Comment: MNRAS, accepted

The stellar origin of gamma‐ray bursts can be explained by the rapid release of energy in a highly collimated, extremely relativistic jet. This in turn appears to require a rapidly spinning highly magnetised stellar core that collapses into a magnetic neutron star or a black hole within a relatively massive envelope. We hypothesize a binary star model that ends with the merging of an oxygen neon white dwarf with the carbon‐oxygen core of a naked helium star during a common envelope phase of evolution, and subsequent accretion induced collapse into a neutron star. The rapid spin and high magnetic field are natural consequences of such a merging. These progenitors, being of intermediate mass, differ radically from the usually postulated high‐mass stars. Thus we can reconcile observations that the bursts occur close to but not within massive star associations.

A simple set of equations is introduced which governs the time evolution of a twisted accretion disc. The time evolution is governed by two «viscosities», one governing shear within the plane of the disc and the other governing shear perpendicular to the disc (brought about by non-planarity of the disc). It is shown that these equations can be put in numerical difference form so that angular momentum is locally conserved. Numerical simulations are given for two simple cases, the evolution of a simple twist in a steady Keplerian accretion disc, and the effect of precession about an axis which is not aligned with the disc on the inner regions of the disc (the Bardeen-Petterson effect)

A model is constructed for the spin-down of rapidly rotating, convective stars. It is assumed that rotation and convection cause a star to rotate differentially, that differential rotation and convection generate a magnetic dynamo, that a magnetic dynamo gives rise to mass loss and a magnetically controlled stellar wind, and that such a wind results in angular momentum loss and hence stellar spin-down. Using the model, it is shown that a protostar accreting from a circumstellar disc reaches an equilibrium rotation period significantly below break-up

We use the properties of OB runaway stars and, assuming that they are ejected from their parent clusters by dynamical binary-binary interactions, deduce certain necessary characteristics of high-mass star formation. We deduce that massive stars form in small groups of binaries that are biased towards unit mass ratio, and that within these groups the IMF is severely under-represented in low-mass stars compared with the field star IMF

The coalescence of supermassive black hole binaries occurs via the emission of gravitational waves, that can impart a substantial recoil to the merged black hole. We consider the energy dissipation, that results if the recoiling black hole is surrounded by a thin circumbinary disc. Our results differ significantly from those of previous investigations. We show analytically that the dominant source of energy is often potential energy, released as gas in the outer disc attempts to circularize at smaller radii. Thus, dimensional estimates, that include only the kinetic energy gained by the disc gas, underestimate the real energy loss. This underestimate can exceed an order of magnitude, if the recoil is directed close to the disc plane. We use three dimensional Smooth Particle Hydrodynamics (SPH) simulations and two dimensional finite difference simulations to verify our analytic estimates. We also compute the bolometric light curve, which is found to vary strongly depending upon the kick angle. A prompt emission signature due to this mechanism may be observable for low mass (10^6 Solar mass) black holes whose recoil velocities exceed about 1000 km/s. Emission at earlier times can mainly result from the response of the disc to the loss of mass, as the black holes merge. We derive analytically the condition for this to happen., 16 pages, accepted by MNRAS. Animations of the simulations are available at http://jilawww.colorado.edu/~pja/recoil.html

The low-mass X-ray binary microquasar GRO J1655-40 is observed to have a misalignment between the jets and the binary orbital plane. Since the current black hole spin axis is likely to be parallel to the jets, this implies a misalignment between the spin axis of the black hole and the binary orbital plane. It is likely the black holes formed with an asymmetric supernova which caused the orbital axis to misalign with the spin of the stars. We ask whether the null hypothesis that the supernova explosion did not affect the spin axis of the black hole can be ruled out by what can be deduced about the properties of the explosion from the known system parameters. We find that this null hypothesis cannot be disproved but we find that the most likely requirements to form the system include a small natal black hole kick (of a few tens of km/s) and a relatively wide pre-supernova binary. In such cases the observed close binary system could have formed by tidal circularisation without a common envelope phase., Accepted for publication in MNRAS

We investigate the evolution of the relative angle between the stellar rotation axis and the circumstellar disc axis of a star that forms in a stellar cluster from the collapse of a turbulent molecular cloud. This is an inherently chaotic environment with variable accretion, both in terms of rate and the angular momentum of the material, and dynamical interactions between stars. We find that the final stellar rotation axis and disc spin axis can be strongly misaligned, but this occurs primarily when the disc is truncated by a dynamical encounter so that the final disc rotation axis depends simply on what fell in last. This may lead to planetary systems with orbits that are misaligned with the stellar rotation axis, but only if the final disc contains enough mass to form planets. We also investigate the time variability of the inner disc spin axis, which is likely to determine the direction of a protostellar jet. We find that the jet direction varies more strongly for lighter discs, such as those that have been truncated by dynamical interactions or have suffered a period of rapid accretion. Finally, we note that variability of the angular momentum of the material accreting by a star implies that the internal velocity field of such stars may be more complicated than that of aligned differential rotation., Accepted for publication in MNRAS, 11 pages, 6 figures

We investigate the relationship between the star formation rate per unit area and the surface density of the ISM (the local Kennicutt-Schmitt law) using a simplified model of the ISM and a simple estimate of the star formation rate based on the mass of gas in bound clumps, the local dynamical timescales of the clumps, and an efficiency parameter of around 5 per cent. Despite the simplicity of the approach, we are able to reproduce the observed linear relation between star formation rate and surface density of dense (molecular) gas. We use a simple model for the dependence of H_2 fraction on total surface density to argue why neither total surface density nor the HI surface density are good local indicators of star formation rate. We also investigate the dependence of the star formation rate on the depth of the spiral potential. Our model indicates that the mean star formation rate does not depend significantly on the strength of the spiral potential, but that a stronger spiral potential, for a given mean surface density, does result in more of the star formation occurring close to the spiral arms. This agrees with the observation that grand design galaxies do not appear to show a larger degree of star formation compared to their flocculent counterparts., 11 pages, 11 figures, accepted by MNRAS

Gas at intermediate temperature between the hot X-ray emitting coronal gas in galaxies at the centers of galaxy clusters, and the much cooler optical line emitting filaments, yields information on transport processes and plausible scenarios for the relationship between X-ray cool cores and other galactic phenomena such as mergers or the onset of an active galactic nucleus. Hitherto, detection of intermediate temperature gas has proven elusive. Here, we present FUV imaging of the "low excitation" emission filaments of M87 and show strong evidence for the presence of CIV 1549 A emission which arises in gas at temperature ~10^5K co-located with Halpha+[NII] emission from cooler ~10^4K gas. We infer that the hot and cool phases are in thermal communication, and show that quantitatively the emission strength is consistent with thermal conduction, which in turn may account for many of the observed characteristics of cool core galaxy clusters., Comment: Accepted for publication in ApJ Letters

Sokoloski et al (2008) have recently reported evidence that the recurrent nova RS Ophiuchi produced a pair of highly collimated radio jets within days of its 2006 outburst. This suggests that an accretion disc must be present during the outburst. However in the standard picture of recurrent novae as thermonuclear events, any such disc must be expelled from the white dwarf vicinity, as the nuclear energy yield greatly exceeds its binding energy. We suggest instead that the outbursts of RS Oph are thermal--viscous instabilities in a disc irradiated by the central accreting white dwarf. The distinctive feature of RS Oph is the very large size of its accretion disc. Given this, it fits naturally into a consistent picture of systems with unstable accretion discs. This picture explains the presence and speed of the jets, the brightness and duration of the outburst, and its rise time and linear decay, as well as the faintness of the quiescence. By contrast, the hitherto standard picture of recurrent thermonuclear explosions has a number of severe difficulties. These include the presence of jets, the faintness of quiescence, and the fact the the accretion disc must be unstable unless it is far smaller than any reasonable estimate., Comment: MNRAS, in press

Be stars are rapidly spinning B stars surrounded by an outflowing disc of gas in Keplerian rotation. Be star/X-ray binary systems contain a Be star and a neutron star. They are found to have non-zero eccentricities and there is evidence that some systems have a misalignment between the spin axis of the star and the spin axis of the binary orbit. The eccentricities in these systems are thought to be caused by a kick to the neutron star during the supernova that formed it. Such kicks would also give rise to misalignments. In this paper we investigate the extent to which the same kick distribution can give rise to both the observed eccentricity distribution and the observed misalignments. We find that a Maxwellian distribution of velocity kicks with a low velocity dispersion, $\sigma_k \approx 15\rm km s^{-1}$, is consistent with the observed eccentricity distribution but is hard to reconcile with the observed misalignments, typically $i \ge 25^\circ$. Alternatively a higher velocity kick distribution, $\sigma_k = 265 \rm km s^{-1}$, is consistent with the observed misalignments but not with the observed eccentricities, unless post-supernova circularisation of the binary orbits has taken place. We discuss briefly how this might be achieved., Comment: Accepted for publication in MNRAS

The possibility of a large, massive protostellar accretion disc playing a role in the formation of binary stars by enabling the capture of a passing star within a dense star-forming region is analyzed. It is found that capture rates are too low to play a major role in all known star-forming environments, particularly when the probability of prior disc dispersal by the more frequent high-velocity interactions is taken into account

The dynamical evolution of a small number of protostars formed by the fragmentation of a dense molecular cloud core, taking into account in a simplified way the dissipative encounters between the protostars caused by the presence of their concomitant circumstellar discs, is analysed. For an initial system of two stars, the resulting binary parameters are determined essentially by the initial angular momentum and to a lesser extent by the rate at which the accretion discs evolve. For an initial system of three stars, however, a narrow range of initial conditions leads to a broad spread of final configurations

The properties of external accretion discs (discs with a central source of angular momentum) are explored both analytically and numerically. An illustrative example of the effect of a disc of material around a binary star on the stellar separation is considered. A Greens-function-type solution is considered, in which an initial ring of matter is put in orbit around the central binary.

In this paper we revisit the arguments for the basis of the time evolution of the flares expected to arise when a star is disrupted by a supermassive black hole. We present a simple analytic model relating the lightcurve to the internal density structure of the star. We thus show that the standard lightcurve proportional to $t^{-5/3}$ only holds at late times. Close to the peak luminosity the lightcurve is shallower, deviating more strongly from $t^{-5/3}$ for more centrally concentrated (e.g. solar--type) stars. We test our model numerically by simulating the tidal disruption of several stellar models, described by simple polytropic spheres with index $\gamma$. The simulations agree with the analytical model given two considerations. First, the stars are somewhat inflated on reaching pericentre because of the effective reduction of gravity in the tidal field of the black hole. This is well described by a homologous expansion by a factor which becomes smaller as the polytropic index becomes larger. Second, for large polytropic indices wings appear in the tails of the energy distribution, indicating that some material is pushed further away from parabolic orbits by shocks in the tidal tails. In all our simulations, the $t^{-5/3}$ lightcurve is achieved only at late stages. In particular we predict that for solar type stars, this happens only after the luminosity has dropped by at least two magnitudes from the peak. We discuss our results in the light of recent observations of flares in otherwise quiescent galaxies and note the dependence of these results on further parameters, such as the star/hole mass ratio and the stellar orbit., Comment: 10 pages, 10 figures, MNRAS accepted

In the microquasar V4641 Sgr the spin of the black hole is thought to be misaligned with the binary orbital axis. The accretion disc aligns with the black hole spin by the Lense-Thirring effect near to the black hole and further out becomes aligned with the binary orbital axis. The inclination of the radio jets and the Fe$K\alpha$ line profile have both been used to determine the inclination of the inner accretion disc but the measurements are inconsistent. Using a steady state analytical warped disc model for V4641 Sgr we find that the inner disc region is flat and aligned with the black hole up to about $900 R_{\rm g}$. Thus if both the radio jet and fluorescent emission originates in the same inner region then the measurements of the inner disc inclination should be the same., Comment: Accepted for publication in MNRAS

The microquasar GRO J1655-40 has a black hole with spin angular momentum apparently misaligned to the orbital plane of its companion star. We analytically model the system with a steady state disc warped by Lense-Thirring precession and find the timescale for the alignment of the black hole with the binary orbit. We make detailed stellar evolution models so as to estimate the accretion rate and the lifetime of the system in this state. The secondary can be evolving at the end of the main sequence or across the Hertzsprung gap. The mass-transfer rate is typically fifty times higher in the latter case but we find that, in both cases, the lifetime of the mass transfer state is at most a few times the alignment timescale. The fact that the black hole has not yet aligned with the orbital plane is therefore consistent with either model. We conclude that the system may or may not have been counter-aligned after its supernova kick but that it is most likely to be close to alignment rather than counteralignment now., Comment: Accepted for publication in MNRAS

The condition in which a protostellar disk must be set up in order to generate FU Ori outbursts is examined. It is found that FU Ori behavior can be produced by perturbation of a low-accretion-rate disk around a normal T Tau star. Suitable perturbations involve a substantial surface-density enhancement and heating to about 10,000 K of the inner disk.

We consider the shape of an accretion disc whose outer regions are misaligned with the spin axis of a central black hole and calculate the steady state form of the warped disc in the case where the viscosity and surface densities are power laws in the distance from the central black hole. We discuss the shape of the resulting disc in both the frame of the black hole and that of the outer disc. We note that some parts of the disc and also any companion star maybe shadowed from the central regions by the warp. We compute the torque on the black hole caused by the Lense-Thirring precession and hence compute the alignment and precession timescales. We generalise the case with viscosity and hence surface density independent of radius to more realistic density distributions for which the surface density is a decreasing function of radius. We find that the alignment timescale does not change greatly but the precession timescale is more sensitive. We also determine the effect on this timescale if we truncate the disc. For a given truncation radius, the the timescales are less affected for more sharply falling density distributions., Accepted for publication in MNRAS